FIG. 1 illustrates a prior art circuit for controlling the flow of AC power to a load. An AC power switch, shown here as a Triac TR, is series connected between the power source 12 and the load 14. A controller 16 operates the Triac to regulate the RMS power transferred to the load using a phase control technique as shown in FIG. 2.
The voltage available from the AC power source is shown as a broken line in FIG. 2. Each line cycle of the AC power source has a positive half cycle beginning at a first zero crossing at time t0 and ending at a midpoint zero crossing at t2. The AC line cycle then has a negative half cycle beginning at t2 and ending at another zero crossing at t4. For common 60 Hz power, the entire line cycle from t0 to t4 lasts 1/60th of a second.
At the beginning of the line cycle illustrated in FIG. 2, the Triac remains off during a delay period TD. At time t1, the Triac is turned on to connect the power source to the load. The portion of the AC voltage waveform actually applied to the load is shown as a solid line. The Triac continues conducting power to the load during conduction period TC until it switches off at the zero crossing at t2. Triacs are self-commutating devices, which means that they turn themselves off when the current through the device falls below a holding level as described below. The same process is repeated for the negative half cycle where the Triac turns on at t3 and off at t4. Generally, if the load is purely resistive, the current IL flowing through the load has essentially the same waveform as the portion of the AC voltage applied to the load as shown in FIG. 3.
By varying the conduction period TC, the amount of power delivered to the load may be regulated. If the load is a lighting load, regulating the amount of power controls the brightness of the load. The waveforms shown in FIGS. 2 and 3 represent a relatively low power setting where a small percentage of the power available during each line cycle is delivered to the load. FIGS. 4 and 5 illustrate a relatively high power setting where the conduction period TC is longer. The greater the area of the solid waveform, the greater the percentage of available power delivered to the load
The time periods illustrated in FIGS. 2-5 are often described in terms of angles where an entire AC line cycle is 360 degrees. Thus, the conduction period TC is commonly referred to as the conduction angle θC, while the delay period TD is typically referred to as the firing angle θF, or the delay angle or triggering angle. The illustrative firing angle in FIGS. 2 and 3 is about 120 degrees, while the illustrative firing angle in FIGS. 4 and 5 is about 60 degrees.